Depends on observation height and the height of the object being viewed. See here: https://dizzib.github.io/earth/curve-calc/?d0=30&h0=10&unit=imperial

Has anyone ever done this experiment on the Bonneville salt flats in Utah? Not sure how far it is from farthest point to the opposite end but it seems that it should be far enough to measure a drop in curvature (if there is one, not sure)with a high powered zoom or whatever.

Not at Salt Flats, but its been done from shore to shore across a few lakes and there was no drop.

The calculator on that website doesn't match reality.  :roll-laugh2:

In Chambers’ Journal, February 1895, a sailor near Mauritius in the Indian Ocean reported having seen a vessel which turned out to be an incredible 200 miles away! The incident caused much heated debate in nautical circles at the time, gaining further confirmation in Aden, Yemen where another witness reported seeing a missing Bombay steamer from 200 miles away. He correctly stated the precise appearance, location and direction of the steamer all later corroborated and confirmed correct by those onboard. Such sightings are absolutely inexplicable if the Earth were actually a ball 25,000 miles around, as ships 200 miles distant would have to fall approximately 5 miles below line of sight!

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This is rich.
So to find a report of sighting a ship 200 miles away you've got to go back 123 years?!
But if the earth were "flat" you should be seeing these things all the time.
All along the coastlines all over the world ships should be visible out at see for thousands of miles.
There should be ships to see to your heart's content on every horizon all around the globe!
Just zoom in with a telephoto lens and make it happen.
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In fact, why stop at 200 miles?
Over a "flat" earth you should be able to see unlimited distances, especially from high elevations.
You say the sun shines from 3,000 miles and there's no problem seeing that at a 45-degree angle, morning and evening.
That means the sun is visible from 4,242 miles every day, so we ought to be able to see Hawaii from California, no problem.
Especially the rising smoke from their erupting volcano lately!
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We ought to be able to see Cuba from Florida and Florida from Cuba, no problem.
From the Galapagos Islands the Andes mountains should be easily seen and obvious.
We ought to be able to see Africa from Italy or Spain.
Portugal and Morocco ought to be in plain sight from the Azores.
We ought to be able to see the Swiss Alps from Alexandria.
We ought to be able to see Iceland from Ireland.
Sightings of land across the Great Lakes and the Caspian Sea should be every day and commonplace.
Japan from Guam and Guam from Formosa! Why not?
From one end of the Philippines to the other sightings across the ocean should be no problem, in every direction.
From the shores of Australia one ought to see New Zealand or Indonesia every day of the week, if the earth were "flat."